In intervention (intravascular treatment) or angiographic examination, the user inserts a catheter into a blood vessel from, for example, a region near the groin and advances the catheter to a desired position. The user performs the operation of advancing the catheter or a guidewire going forward through the catheter to a desired position under radiographic fluoroscopy. In general, no blood vessel can be seen on an X-ray fluoroscopic image. For this reason, the user grasps the position of the catheter or guidewire in the human structure by seeing an X-ray fluoroscopic image and operates the catheter or guidewire by using knowledge concerning a blood vessel structure at the position. If, however, a blood vessel structure is complicated and it is difficult to insert the catheter or guidewire, it is necessary to enhance the contrast of blood vessels with a contrast medium or the like under X-ray fluoroscopy. However, if injection of a contrast medium into a patient is continued, it may lead to renal dysfunction or the like.
For this reason, the user uses a road map in intervention or angiographic examination. Traditional 2D road mapping is a function of displaying, as a road map image, a DSA (Digital Subtraction Angiography) image obtained by subtracting object images before and after the injection of a contrast medium from each other while superimposing the image on a real-time X-ray fluoroscopic image. Although the DSA image is used in this case, it is possible to use other types of images as long as they can enhance the contrast of blood vessels. For example, it is possible to use an X-ray contrast image concerning an object after the injection of a contrast medium. It is also possible to use an image depicting blood vessel centerlines, an image with enhanced blood vessel walls, and the like which are obtained by processing a DSA image and an X-ray contrast image.
The 2D road mapping is a function of displaying a superimposed image obtained by positionally aligning a real-time fluoroscopic image with a 2D road map image. A 2D road map image is an image explicitly showing the running of blood vessels in a patient. For example, a 2D road map image is a DSA image or the like. A DSA image can be acquired routinely. For this reason, 2D road maps are often used in intervention and angiographic examination. However, it is difficult to grasp depth information concerning blood vessels from a DSA image. For this reason, the user changes the observation direction (imaging direction) of blood vessels. Every time the user changes the observation direction of blood vessels, it is necessary to obtain an additional DSA image corresponding to the observation direction after the change. If, therefore, the user frequently changes the observation direction of blood vessels, it requires many tasks, resulting in an increase in the dose of contrast medium injected into the patient.
A 3D road mapping is a function of displaying a 3D road map image and a real-time fluoroscopic image upon positionally aligning them with each other. A 3D road map image differs from a 2D road map image in that it is available to generate 3D road map images observed from any positions or any angles by 3D image processing. A 3D blood vessel image data is formed based on a plurality of DSA images having different imaging angles. A plurality of DSA images having different imaging angles are acquired while one imaging system is rotated through 200° or more around an object at high speed (this operation will be called rotational DSA imaging hereinafter). A 3D blood vessel image data has 3D blood vessel information. Therefore, in 3D road mapping, a 3D road map image is generated from 3D blood vessel image in accordance with (following) a change in observation direction. This allows the user to easily change the observation direction without re-injecting a contrast medium into a patient. In addition, if a 3D road map image is a rendering image, shading is applied to the image. This allows the user to discriminate the anteroposterior relationship (in the depth direction) between blood vessels to some extent.
As described above, 3D road mapping is a function useful from the viewpoint of three-dimensionally grasping a blood vessel structure. However, in order to form a 3D road map image, it is necessary to completely interrupt an operation when acquiring rotational DSA images to generate 3D blood vessel image data. In addition, it takes much time for the acquisition, and requires a further dose of both contrast medium and X-ray exposure. For such reasons, a 3D road map is not used for the purpose of the navigation of a catheter or guidewire in practice. However, previous 3D blood vessel image gives us highly accurate and detailed vessel structures. For navigation purpose, this is much more than necessary level. It should be sufficient to provide the branching angle of blood vessels at bifurcation area for inserting catheter or guidewire into interested branch. There has been proposed no 3D road map from such viewpoint.